Magnetic Resonance Imaging (“MRI”) systems are well known in the art and widely used in medicine. These systems typically include a magnet system for generating a steady magnetic field, a magnet system for generating gradient fields, and an RF transmitter coil for generating an RF field which excites nuclei in a patient for magnetic resonance. The magnetic resonance signal is detected by the RF transmitter coil or by a special RF receiver coil or coils.
Three important parameters to evaluate an RF receive coil are signal-to-noise ratio (SNR), homogeneity, and field-of-view (“FOV”) coverage. The SNR may be defined as the ratio between the signal strength on the image and the background noise. An RF coil typically achieves a higher SNR when it is closer to the part of the patient being imaged.
The homogeneity measures signal sensitivity variation in the RF coil. The receiving sensitivity of the RF receiver coil generally decreases with increasing distance from the coil wires.
The FOV refers to the region required to be covered by the RF coil. A large RF coil such as the body coil provides a large FOV coverage, but the MRI signals detected with such a coil have low SNR. By contrast, a small RF coil provides only a small FOV coverage but has a higher SNR.
A system offering both a higher SNR and larger FOV coverage is disclosed in U.S. Pat. No. 4,825,162 to Roemer. The Roemer patent teaches an RF coil construction for an MRI apparatus. The Roemer coil construction includes several single-loop coil elements arranged in a phased array to increase the FOV coverage and SNR of the images. The coil elements overlap each other and include tuning and matching capacitors.
The invention described in the Roemer patent has disadvantages. For example, the SNR of the RF coil construction in the Roemer patent is limited by the SNR of its individual coil elements. Consequently, there is a need in the art for an alternating system that further increases its SNR by increasing the SNR of its coil elements.